A capacitor microphone is a sort of electroacoustic converters which catch a mechanical displacement of a vibration plate vibrated by a sound wave as the variation of electrostatic capacitance and convert the variation of the electrostatic capacitance to an electric signal. In a prior art, a capacitor microphone provides a vibration plate (diaphragm) 10 and a charge back-plate (fixed charge-plate) 20 as shown in a sectional view of FIG. 9 and in an exploded-perspective view of FIG. 10.
The vibration plate 10 is composed of a thin film such as polyphenylene sulfide (PPS) and is strained with a given tensile force and fixed on one end-face of a support ring (diaphragm ring) 11. The charge back-plate 20 is composed of matter such as an electret board and is fixed on one end-side of a cylinder base 21.
A spacer 30 is arranged between the vibration plate 10 and the charge back-plate 20 in order to form a capacitor. A plastic film such as polyethylene terephthalete (PET) punched in the shape of a ring is mostly used as the spacer 30.
Since the capacitor microphone has the directivity of an unidirectional polar pattern, rear acoustic terminals 21a are formed on the cylinder base 21. In the charge back-plate 20, through-holes 20a are punched in order to communicatively connect the rear acoustic terminals 21a to the reverse face side of the vibration plate 10.
As described above, in the capacitor microphone the capacitor is formed by disposing the spacer 30 between the vibration plate 10 and the charge back-plate 20. The capacitor includes both of an effective electrostatic capacitance which serves the generation of the electric power and a noneffective electrostatic capacitance which does not serve the generation of the electric power, and the greater is the effective electrostatic capacitance of the capacitor, the better is the signal-to-noise ratio of the capacitor microphone.
The noneffective electrostatic capacitance (stray electrostatic capacitance), which causes a capacitor microphone to reduce a gain, exists at the contact part of the spacer 30 and the charge back-plate 20. In the capacitor microphone having a comparatively greater diameter, it will be possible that a noneffective electrostatic capacitance is designed to be smaller relatively to an effective electrostatic capacitance.
However, in a capacitor microphone having a smaller diameter, particularly, in an electret capacitor microphone in which FEP should to be laminated on a charge back-plate, it is difficult to decrease the noneffective electrostatic capacitance. Especially, since in a microphone having a smaller diameter, such as a lavalier microphone (tie pin microphone), the effective electrostatic capacitance is originally smaller, the gain is considerably decreased by the noneffective electrostatic capacitance.